How do you get Real bass sound (no guitarish twang)on the G string tuned to A ?

I am looking for string that will give me that fundamental bass sound on my G string. I've made the task more complicated because I would like to tune the G to A so I can tune in fifths (cello tuning / CGDA). Everything I have tried sounds twangy/guitarish to me. I've tried 40's and 45's. For what I am looking for the 45's are better. 50's tuned to A would be more that 60 lbs of tension which to date has frightened me off. I know the 'Jameson Set' has the G at 52 but James didn't tune to A. I've tried D'Addario XL and slow wounds, Ken Smith pressure wounds and half rounds, and GHS and Thomastik Flats. For what I am looking for half-rounds are better than rounds and flats better than half rounds but I'm still not where I want to be. I test these on passive with no attenuation on my amp. I would like the fundamental sound to be there without teaking. Obviously cutting the highs and boosting the lows and mids helps but even after I switch to active, tweak my bass and teak on my amp I'm still not getting what I am looking for. Am I beating my head/ears against the laws of physics ? (I have a relatively new Carvin LB-70 and SWR Basic Black (15" with the tube preamp).)
Thanks for you thoughts and suggestions.

I've always had a hard time getting a fat sound out of the G. I'm currently working on overcoming this with technique as much as string selection. By using the meatiest part of my fingertip and plucking pretty hard, I'm getting there. It works on either flatwounds or (nickel) rounds. I was getting a huge sound out of some Elixirs, but I've switched back to flats just because I like them better. The fattest sounding G string I've ever had is a Thomastik-Infeld Jazz Flatwound. These are kind of thin (.043?) and low-tension. I'm not using TI's now because I didn't like the E string, but the G is pretty thumpy.

Fat strings don't necessarily equal a fat sound. I tried tuning a .060 D'Addario Chrome string to G, and it wasn't any better IMO than a .045 Fender flat. However, one thing I like about fat strings is higher tension, which allows you to hit the string harder for a big thump, without fret rattle.

I would think, though, if you used Thomastiks, while the G would probably work at A, the E would be pretty wobbly tuned down to C.

Ok heres what i think. For the low C you could either buy a 5 string set then get an additional string to tune up to A or just use that set [and get rid of the stock e string] , or buy a low b string [tune up to c] separately & replace the e string on your bass. For the A string try checking with orchestra instrument stores & asking them if they know what gauge the A string is. A custom set composed of separately bought gauges might be the way your gonna end up going. The low e would be to loose to play if you just tuned down. I think the idea of using cello tuning is a great one. Thats all